Comparison of acetylcholine receptor-controlled cation flux in membrane vesicles from Torpedo californica and Electrophorus electricus : Chemical kinetic measurements in the millisecond region

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Abstract
AcChoR-controlled cation flux was measured in T. californica vesicles by using a pulsed-quench-flow technique with a 2-ms time resolution. Torpedo vesicles on a weight basis may contain several hundred times more receptor sites than do E. electricus vesicles. Techniques have been developed to correct for the kinetic heterogeneity of the vesicle population; to use the inactivation of the receptor by its natural ligand to reduce influx rates at high ligand concentrations to a measurable level (this permitted JA, the influx rate coefficient before the onset of inactivation, to be measured); and to determine the rate coefficients of 2 processes that lead to successive inactivations (desensitization) of the receptor and occur in different time regions. An extension of a model proposed for the E. electricus receptor accommodates the ion translocation in T. californica vesicles. The features in common are a rapid initial flux rate [JA(max) for T. californica is 310 s-1; for E. electricus it is 7.5 s-1], these differences in flux rates being consistent with a difference in AcChoR density; a rapid inactivation process [.alpha.(max) for T. californica is 2 s-1; for E. electricus it is 7 s-1]; and a slow AcChoR-controlled flux that continues after the rapid inactivation [JI(mas) for T. californica is 1.3 s-1; for E. electricus it is 0.015 s-1]. The main difference between the flux in the 2 types of vesicle is the existence of a 2nd, slower, inactivation process in T. californica with a rate coefficient, .beta., of 0.12 s-1. The 2nd process leads to undetectable flux activity during the time of observation (30 s in 10 mM carbamoylcholine). These studies are also significant because fundamental differences may exist between the mechanism of AcChoR-controlled ion flux in synaptic (Torpedo) and conducting (E. electricus) membranes.

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